When the power supply voltage fluctuates and falls below a certain voltage during operation of a semiconductor chip, such as an LSI, for example, an input/output interface circuit might experience an operational failure. Examples of the cause of a significant reduction in power supply voltage include the following cases: defective design of the semiconductor chip, which causes high current to flow locally, resulting in a voltage drop; and partial disconnection of wiring between a terminal of the power supply and a terminal of the semiconductor chip during packaging, which causes the power supply voltage to drop by more than a design value.
Inspection of the fluctuations in power supply voltage in the semiconductor chip is necessary to confirm whether there is any defect in design of the semiconductor chip, or to analyze any operational failure due to wiring disconnection or suchlike. At present, fluctuations in power supply voltage in the semiconductor chip are detected using a specialized measurement apparatus after processing the semiconductor chip with a specialized processing apparatus to expose its circuit portion and place needles in wired portions.
However, such a detection method takes time to process the semiconductor chip, and requires expensive apparatuses for processing and voltage measurement, resulting in increases in development period and cost of semiconductor devices. Furthermore, the detection method has a problem in that semiconductor chips are processed as samples, hence not usable thereafter.
In view of the circumstances as described above, there is some demand for a device capable of detecting fluctuations in power supply voltage during an operating state of a semiconductor chip without using any specialized apparatuses, while minimizing increases in cost of developing and producing semiconductor chips, as well as enabling reuse of the semiconductor chips used as samples.
To satisfy the aforementioned demand, there has been proposed a semiconductor device having incorporated therein a voltage fluctuation detection device composed of simple circuits (see U.S. Patent Application Publication No. US 20020196048 A1).
FIG. 21 illustrates the configuration of the semiconductor device 50 described in the above prior art document. The semiconductor device 50 includes a normal circuit 51, and a voltage fluctuation detection device 52. The voltage fluctuation detection device 52 includes an inverting amplifier 53 and a latching circuit 54. The inverting amplifier 53 is connected between a terminal 521 for an external power supply 500 that supplies power to the normal circuit 51 and a terminal 522 to which a ground potential VSS is supplied. The latching circuit 54 has a reset terminal connected to an output terminal of the inverting amplifier 53.
Note that the normal circuit 51 refers to one of the circuits included in the semiconductor device 50 that is used during normal operation and has any portions used only for inspection excluded therefrom. In addition, the potential of the external power supply 500 is denoted by VDD, and the power supply voltage of the normal circuit 51 is denoted by Vc. The potential VDD of the external power supply 500 normally shows a constant value, whereas the power supply voltage Vc applied from the external power supply 500 to the normal circuit 51 changes in accordance with the operation of the normal circuit 51. The same applies to the following descriptions.
When detecting the power supply voltage Vc of the normal circuit 51, a reference voltage VDC is supplied from an external power supply (not shown) to an input terminal 55 of the inverting amplifier 53. When the power supply voltage Vc falls below the value of the reference voltage VDC supplied to the terminal 55, an output from the latching circuit 54 is inverted. Accordingly, based on the output from the latching circuit 54, it is possible to detect whether the power supply voltage Vc is below the reference voltage VDC.
According to the conventional semiconductor device as described above, by repeating voltage detection while changing the value of the reference voltage VDC, it becomes possible to detect the range of fluctuations in the power supply voltage Vc during the operation of the normal circuit 51. Therefore, by using the detection results, it becomes possible to eliminate any semiconductor chip that might be defective in terms of operation at the time of product shipment or suchlike.
However, the above conventional device is not capable of detecting the value of the power supply voltage Vc at arbitrary times during the operation of the normal circuit 51. In other words, any fluctuations in power supply voltage over time cannot be detected, and therefore there is a limit in the ability to analyze the behavior of semiconductor devices.
In addition, the above conventional device requires wiring for analog signals because an analog signal is inputted from the terminal 55 as the reference voltage VDC. Since the wiring for analog signals is susceptible to influences of resistance and noise compared to wiring compatible with digital signals, it is necessary to design the device considering such aspects, which leads to a relative increase in size. As a result, the wiring for detecting voltage fluctuations disadvantageously imposes constraints on the design of semiconductor chips.